PHYSIOGRAPHY OF THE PHILIPPINE ISLANDS: II. 13 



in depth extending as far as Gadd Eock, about 31 Idlometers soutla of the 

 island of Little Betel Tobago, where it ends sharply in the deep Bashi 

 Channel. Between Botel Tobago and Formosa, on the other hand, a 

 depth of 1,050 fathoms (without bottom) is reached. From Formosa 

 a similar ridge extends southwards for about 90 kilometers where a 

 sounding of 73 fathoms is recorded, but be3rond this there seems to be a 

 marked deepening to the southward. The "position doubtful" Mark Lane 

 Shoal in latitude 21° north, longitude 120° east may be a part of this 

 same shelf. A line of deep soundings through the Bashi Channel gives 

 the following depths: 2,645 fatlioms, 55 kilometers east-northeast of 

 Botel Tobago; 2,618 fathoms, 40 kilometers east of Little Botel Tobago; 

 1,287 fathoms, 27 kilometers east-southeast of Gadd Eock ; 1,009 fathoms, 

 midway between Gadd Eock and Y'Ami; 2,009 fathoms, 40 kilometers 

 west of Y'Ami; 2,053 fathoms, the same distance west of Isbayat; 1,784 

 fathoms, 55 kilometers west-southwest of Isbayat, and 936 fathoms, 73 

 kilometers west-southwest of Desquey. These soundings seem to show the 

 existence of a deep trough connecting the "deep" off the west coast of 

 northern Luzon with that to the soiitheast of tire Eiukiu Islands. 



On the Americaji side of the Baslii Channel the soundings are those 

 made by the Britisli survey of 1845 and are for the most part "without 

 bottom" and hence of less value for our purposes. Ko soundings are 

 given in the vicinity of Isbayat and the Siayanes, except between Y'Ami 

 and May sanga where there is a minimum depth of 36 fatlioms, probably 

 from tidal scour and recent depression. 



Between Ibujos and Sabtan there are for some reason a great number 

 of soundings, generally less than 20 fathoms. The most interesting 

 feature of these is a narrow trough of from 19 to 45 fathoms depth 

 close to the northern part of the west coast of Sabtan. Another feature 

 is the sudden deepening beyond 20 fathoms, the 30- and 30-fathom 

 countours being in most cases close together. There are very few sound- 

 ings to bottom near Batan Island, but such as are given seem to indicate 

 the existence of a 20-fathom shelf. 



GEOLOGIC PEOBLEMS. 



Having given a general description of the islands it seems advisable 

 brieiiy to touch on the chief geologic problems before proceeding to a 

 pliysiographic discussion. 



THE ORIGIN OF THE AGGLOMEBATE. 



The volcanic agglomerate, so far as we can see, is the basal roclc of 

 these northern islands. In thin sections of limestones which rest upon 

 it, Mr. W. D. Smith has found the Miocene fossils Orlitoides and 

 Litliothamnium, hence the agglomerate may he considered pre-Miocene. 

 Thus, in accounting for its origin, hj^potheses depending on existing 

 physiographic forms, such as the explosive activity of Mount Iraya or 



